In general, much of the developmental work of communication systems focused on voice service, and communication systems have also been developed to additionally provide data service and a variety of multimedia services beyond the voice service. The voice-oriented communication systems have not fulfilled increasing user demands concerning services due to a relatively narrow bandwidth and a high service charge. Moreover, the development of the communications industry and an increasing demand for Internet service have been a driving force behind the need for a communication system that efficiently provides the Internet service. In this context, a BWA system has been proposed, for providing the Internet service efficiently in a broad band enough to satisfy the ever-increasing user demands. The BWA communication system is under standardization mainly in the Institute Electrical and Electronics Engineers (IEEE)802.16 working group.
As compared to conventional wireless technologies for voice service, owing to a broad band, the IEEE 802.16 standards enable transmission of more data for a shorter time and allow all users to share channels or resources, for efficient use of the channels or resources. In addition, since Quality of Service (QoS) is guaranteed, services with different QoS requirements can be provided to users according to the characteristics of their services.
The BWA system adopts Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) for physical channels. In OFDM/OFDMA, a physical channel signal is transmitted on a plurality of subcarriers, thus making high-speed data transmission possible. Meanwhile, the BWA system supports CSM by allowing two mobile stations to transmit uplink signals in the same resources.
Conventionally, an uplink scheduler prioritizes mobile stations and allocates radio resources first to a mobile station (MS) with the highest priority level. The scheduler also controls inter-cell interference to a predetermined level by adjusting a Modulation and Coding Scheme (MCS) through interference control. Without CSM, one slot is allocated to one MS and thus no interference exists within the same sector (or cell).
On the other hand, use of CSM causes intra-sector interference because two mobile stations use the same slot in different pilot patterns. What is worse, if a neighbor sector also uses CSM, interference from the neighbor sector increases twice. A conventional scheduler allocates resources to mobile stations using first and second pilot patterns irrespective of their locations. Hence, the same slot can be allocated to two mobile stations at a cell edge, thereby further increasing interference to other sectors.
When system load (e.g. Rise Over Thermal (RoT)) increases, the scheduler allocates a low MCS level to an MS in order to keep the load at an acceptable level. If the use of CSM increases the system load to or above a predetermined level, the scheduler decreases MCS levels for all mobile stations. In this case, even though CSM doubles the number of available slots, the overall system throughput decreases.
Table 1 compares Single Input Multiple Output (SIMO) with CSM in terms of system throughput (TP), RoT, and MCS level distribution.
TABLE 1SIMOCSMTP1805159.901691563.29RoT5.015.50MCS distributionQPSK 1/1200QPSK ⅛23275318609QPSK ¼1859348972QPSK ½202960220537QPSK ¾0016QAM ½1606811168517716QAM ¾547513864248
As noted from Table 1, compared to SIMO, CSM increases RoT but decreases TP. When CSM is based on interference control, more of lower MCS levels are distributed than in SIMO.
As described above, if the CSM system performs scheduling in the conventional manner, system throughput decreases despite a twofold increase in available resources.